Here Come the Perseids!

August 9, 1999:
The attention of the world will be riveted on the heavens this week as the
last total solar eclipse of the 20th century takes place on August 11.
However, for most skywatchers outside the path of totality, the best sky show
won't occur until a day and a half later, on Thursday, August 12, and Friday,
August 13. That's when the annual Perseid meteor shower reaches its peak with
50 to 150 shooting stars per hour.

Right: A Perseid meteor from 1993. The
colors are representative but digitally enhanced. As the meteor streaked
across the night sky, different excited atoms emitted different colors of
light. The origin of the green tinge visible at the right is currently
unknown, however, and might result from oxygen in Earth's atmosphere. Credit
& Copyright: S. Kohle & B. Koch (Astron. I., U. Bonn) [more information]

In recent years the spectacular
Leonid meteors have attracted considerable attention, but historically
the Perseids are the best known of all meteor showers. It rarely fails to
provide a pleasing display and, because of its summertime appearance, it
tends to attract many astronomy novices.

This year should be a good one for viewing Perseids. The shower's maximum
takes place on August 12, 1999, under dark skies just one day after the new
moon. Although the Perseids officially begin in late July, the shower builds
rather slowly in intensity until there is a series of sharp peaks in activity
near mid-August.

The figure below shows the expected activity profile for 1999 based on
observations in 1997. The shower's maximum is centered around 21 UT (2 p.m.
PDT) on Thursday, August 12, 1999. The level of activity remains relatively
high for nearly 24 hours afterward, so the early morning hours before dawn on
Thursday, August 12 and on Friday, August 13 should be good times to observe
in most places [click for more observing tips]. The
other two times indicated in the figure (0700 UT on August 12 and August 13)
are launch windows for the Science@NASA Perseids
Live! high altitude balloon flight which will transmit a live webcast of
the meteor shower from the stratosphere.

Left: 1997 Perseids activity based
on visual records from The International
Meteor Organization. The vertical axis is the "zenithal hourly
rate" of visual meteors, or the hourly rate of meteors an observer
would witness under ideal conditions with the meteors appearing directly
overhead. The horizontal axis is the solar longitude of Earth, and may also
be regarded as time increasing from left to right. If the activity profile
in 1999 is similar to 1997, then maximum activity will occur around 2100 UT
on August 12, 1999

Like most meteor showers, the Perseids are
caused by comet debris. As comets enter the inner solar system, they are
warmed by the sun and peppered by the solar wind, which produces the familar
tails that stretch across the night sky when a bright comet is close to
Earth. Comet tails are made of tiny pieces of ice, dust, and rock which are
spewed into interplanetary space as they bubble off the comet's nucleus. When
Earth encounters these particles on its journey around the Sun, they strike
the atmosphere speeds exceeding 100,000 mph. (The average speed of Perseid
meteoroids is 130,000 mph!) Most are observed as a bright streak across the
sky that can last for several seconds, but occasionally a large fragment will
explode in a multicolored fireball. Most of the streaks (popularly called
'shooting stars') are caused by meteoroids about the size of a grain of sand,
but much less dense. Although they travel at high speeds, these tiny
meteoroids pose no threat to people or objects on the ground.

The Perseids were the first meteors ever associated with a particular
comet. From 1861 to 1863, observers noted a great increase in the number of
August Perseids. As many as 215 per hour were seen in 1863. The Italian
astronomer Giovanni Virginio Schiaparelli (better known for giving the name
"canali," or "channels," to the dark linear markings on
Mars) calculated the orbits of some Perseid meteoroids and discovered that
they closely matched that of periodic comet Swift-Tuttle,
which had been discovered in 1862 during its close approach to Earth.
Swift-Tuttle orbits the Sun once every 135 years. The last time it passed
near Earth was in December 1992. The proximity of the comet once again caused
an increase in Perseid activity and, in August 1993, observers in Central
Europe were treated to 200 to 500 meteors per hour. Swift-Tuttle won't make
another swing through the inner solar system until 2126, but when it does the
comet itself is expected to be an impressive sight as seen from Earth,
rivalling Comet Hyakutake in 1996 or Comet Hale-Bopp in 1997.

Left: Comet Swift-Tuttle, shown here
in false color, is the largest object known to make repeated passes near the
Earth. It is also one of the oldest known periodic comets with sightings
spanning two millennia. Last seen in 1862, its reappearance in 1992 was not
spectacular, but the comet did become bright enough to see from many
locations with binoculars. To create this composite telescopic image, four
separate exposures have been combined, compensating for the motion of the
comet. As a result, the stars appear slightly trailed. The inset shows
details of the central coma. The unseen nucleus itself is essentially a chunk
of dirty ice about ten kilometers in diameter. Credit: D. McDavid (Limber
Observatory), D.C. Boice (SwRI). [more information].

Perseid observing tips

Perseid meteors can be seen anytime after the
sun has set and the constellation Perseus is above the horizon (which is
nearly all the time from observing sites in the northern hemisphere). In
practice, watching for meteors between sunset and midnight is rarely
profitable. The best time to look is between about 2 a.m. and dawn. That's
when the local sky is pointing directly into the meteoroid debris stream (see
the diagram below). The early morning hours of August 12 and August 13 should
be good times to watch if you live in the northern hemisphere.

Above:The
rate of meteor activity is usually greatest near dawn because the earth's
orbital motion is in the direction of the dawn terminator. Earth scoops up
meteoroids on the dawn side of the planet and outruns them on the dusk side.

Perseidsat a Glance

The meteor shower is active
from July 23 until August 22.

Maximum activity is
expected at 2100 UT on August, 12 1999.

The radiant is at RA=3h04m,
DEC=+58o

Average magnitude 2.3

Current Moon Phase

Updated every 4 hours.

For northern hemisphere observers at
latitudes higher than about 35 degrees Perseus is circumpolar -- it is always
above the horizon. Unfortunately, due to the high declination of the radiant,
the Perseids are not a good shower for skywatchers south of the equator.

The sky map below represents a view of the sky looking
northeast from a mid-latitude viewing site at 3:00 a.m. (local time wherever
you live). The radiant, in the constellation Perseus, is located almost
midway between Jupiter in the east and Polaris in the north. Jupiter
(magnitude -2.6) and Saturn (magnitude +0.3) will be very bright and easy to
spot.

Experienced
observers suggest the following viewing strategy: Bring a reclining chair, or
spread a thick blanket over a flat spot of ground. Lie down and look up
somewhat toward the north. You don't need to stare directly at the radiant --
the meteors can appear anywhere in the sky. Their trails will tend to point
back toward the radiant, pictured as a red dot in the sky map below.
Binoculars and telescopes are not essential. The naked eye is usually best
for seeing meteors which often streak more than 45 degrees across the sky.
The field of view of most binoculars and telescopes is simply too narrow for
good meteor observations.

The image indicates the
general region of the sky from which the Perseid meteors appear to emanate
(red dot). This point, called the radiant, is really an optical illusion -
the meteors are moving along parallel paths, but appear to come from a single
point, just as a stretch of parallel railroad tracks will appear to meet at a
point on the horizon.